Recently, large-capacity, high-density, and high-speed memory systems capable of freely processing video information are essential for the future information field. Studies are conducted which create a three-dimensional optical memory inside quartz glass on the basis of the advance of light waveguides and ultrashort-pulse laser beam processing technologies. A laser beam is emitted and condensed inside glass to create spots that have a photo-induced contrast in index of refraction at a focal light point. They are deposited and aligned as bits in three dimensions, and are read out with light using the contrast in index of refractive, in which a capacity two digits larger than before can be expected, but rewriting is not yet possible.
As described above, recently, a demand for a large capacity of information transmission is increasing and research and development of higher-density optical memory elements are popular. As a method for achieving higher density of recording media, attempts are being made to make a recording region from a two-dimensional one to a three-dimensional one. There is provided a photochromic memory that uses photochromatism of a photochromic molecule. Although it is a rewritable memory because of its reversible reaction, there is a problem in thermal stability and repetitive durability because of the chromatic reaction of an organic material.
On the other hand, a light-guide manufacturing technique and a laser-beam processing technique have made a remarkable advance, recent papers report creation of a three-dimensional optical memory inside a stable glass material. When a transparent material such as glass is irradiated with condensed ultrashort light pulses, the vicinity of the focal light point has photo-induced contrast in index of refraction, having a different index of refraction from the surroundings to create bits. Such bits are arranged in a three-dimensional array.
Such conventional arts include the following:
(1) Japanese Unexamined Patent Application Publication No. 8-220688
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